Mohs micrographic surgery is a surgical technique for removing tumorous cells involving a repeated series of surgical excisions followed by microscopic examination of the tissue to assess if any tumorous cells remain. Essentially, the examination of the tissue during the procedure informs the surgeon where to remove tissue next and the surgeon performing the procedure is generally also the pathologist reading the tissue specimen slides.
Mohs surgical procedure typically involves the following steps, represented by FIGS. 1A to 1G. In step 1A, narrow resection margins are made around a grossly identifiable tumor. In step 1B, notches are made in the tissue sample and in surrounding normal tissue for conserving orientation during the procedure. In step 1C, the resected tumor is prepared in such a way that the complete resection margin is presented in one plane. At this point, colored inks are used to identify the notches. In step 1D, a microscope slide is made from the prepared sample that shows the path of the surgeon's knife. In step 1E, the slide is interpreted under a microscope for the presence of tumorous cells. In step 1F, the areas having tumorous cells are mapped on a graphic representation of the surgical site. In step 1G, the surgeon follows the map and removes remaining tumorous areas, where needed. The steps are repeated until no tumorous cells remain.
The goal of Mohs surgery is to completely remove all tumor tissues with the minimal amount of tissue loss. By examining complete on face tissue margins, the surgeon can be confident that the tumor is completely excised. By removing the tissue in multiple, very thin layers, the point at which no tumor remains will become obvious with the least amount of tissue excised.
The Mohs surgeon, also acting as pathologist, relies on the microscope slide prepared by a technician for diagnosis. However, the task of preparing an accurate representation of the resection margin has multiple pitfalls. More particularly, errors of flatness and errors of knife alignment are not resolved by known techniques presented hereinbelow.
In the Cryoembedder technique, also described in U.S. Pat. No. 7,059,139 issued to Marsing et al., a cryoembedder consists of a two-part jig that can accept the cryostat chuck on one part and a smooth disk on the second part. The tissue is frozen with the resection margin down on the smooth disk and the disk is placed on one of the jig faces. The cryostat chuck is placed in the other jig face and covered with embedding medium. When the embedding medium begins to freeze, the two jig faces are brought together and the sample to the chuck is frozen. Once frozen, the jig is separated and the smooth disk is broken free from the chuck-specimen-disk sandwich. The chuck is then ready for sectioning. Unfortunately, the Cryoembedder technique assumes that the object head is parallel to the knife holder, and due to the sliding fit clearance of the alignment pins of the jig, the faces may not always be parallel.
Also known as the Slide technique, the tissue is frozen flat against a glass microscope slide with the resection margin down against the slide. The technician is able to look under the slide to see if the surface of the tissue is making proper contact with the slide and corrections can be made before proceeding with the study. The slide is placed on the freeze bar for freezing. An embedding medium is poured around and over the tissue. A chuck is placed in the freezing bar and covered in embedding medium. As soon as the embedding medium begins to solidify on the slide, the tissue-slide block is flipped over to freeze on top of the chuck. Alternatively, the tissue covered in embedding medium is allowed to freeze completely. The slide is then removed from the cryostat and a warm hand is used under the glass slide to soften the tissue-embedding medium block. The tissue-embedding medium block is then freed from the glass. The tissue-embedding medium block is placed back in the cryostat and the chuck is placed in the freeze bar. The embedding medium is added on top of the chuck and as it freezes, the tissue-embedding medium block is placed on the chuck resection margin side up. However, one drawback with the Slide technique is that the tissue is frozen flat but not aligned with the knife.
In the Miami Special technique, a plier style (scissor, clamp) tool with wide jaws is used. One of the jaws has a hole or slot for accepting a chuck. Tissue is frozen flat using any technique the technician prefers, such as in the Slide technique described above. The tissue-embedding medium block is freed from the slide and placed against the smooth jaw. A chuck with embedding medium is placed on the modified jaw and the two jaws are brought together by squeezing the handles together. The jaws are then submerged in liquid nitrogen to freeze the tissue-embedding medium block solid. The jaws are removed from the liquid nitrogen, opened and the chuck is removed for use. However, drawbacks of the Miami Special technique relate to the need for liquid nitrogen and the angle that the jaw closes at sets the plane of the resection margin in relation to the chuck. Thicker specimens are set at a greater angle than thinner specimens.
Many cryostat manufacturers include a heat extractor with their units in which a mobile heat sink device is used for flattening and speeding the freezing of specimens. In the Heat Extractor technique, the heat extractor is essentially a chrome-plated metal weight with a smooth flat underside that is kept in the cryostat in order to keep it cooled. In previous practices, a chuck with embedding medium was placed on the freeze bar. The tissue was placed on top of the solidifying embedding medium and then the heat extractor was place on top of the tissue to flatten and freeze it. In recent years, Mohs technicians have begun using the flat underside in the same manner as done in the Slide technique. However, the tissue quickly freezes to the cold metal surface. Once frozen, the tissue is covered with embedding medium and a chuck is prepared on the freeze bar with embedding medium. The two are brought together and the block quickly freezes. One drawback of the Heat Extractor technique is that it is hard to get the tissue to freeze at the correct angle to match the angle of the knife.
In the Precision Cryoembedding System technique, also described in US 2002/0162337 A1 originally assigned to Stephen Peters, there is a metal bar that has a series of shallow flat bottomed wells. The wells are cut with a tapered mill. All the surfaces are smoothly polished and, in conjunction with the sloped sides, the embedding medium is aided in breaking free. The tissue is frozen flat to the bottom of the well and is covered in embedding medium until the well is full. A chuck is placed upside down on top of the well, in order for the peripheral edge of the chuck to rest on the edges of the well, which keeps the face of the chuck parallel to the bottom of the well. Once frozen, the stem of the chuck is jarred and the tissue-embedding medium-chuck block is freed from the mold. The drawbacks associated with the Precision Cryoembedding System technique are that not all chucks work with this system, as some chucks are stemless, and it is not possible to compensate for the knife angle as the tissue is parallel to the chuck face.
In the Plastic Molds technique, similar to that of the Precision Cryoembedding System but with thin plastic molds, the transparent molds freeze slower and allow the technician to look under the mold to see if the tissue is making proper contact in the center of the mold. A chuck can be frozen to the top of the mold or the tissue block can be popped out from the mold and mounted using any other technique known in the art. However, some drawbacks of the Plastic Molds technique are slow freezing; the molds are too small for larger specimens and difficult to compensate for knife angles.
Other techniques known in the art, such as the technique described in U.S. Pat. No. 5,776,298 issued to Franks, have drawback similar to the techniques described above.
Thus, known techniques for microtomy in the field of Mohs micrographic surgery lack the full set of attributes needed for minimizing tissue loss in sectioning when the tissue is aligned on a chuck, while minimizing errors of flatness and errors of knife alignment. There therefore exists a need in the art for a tissue aligning tool and method for preparing a tissue plane on a chuck for microtomy which alleviates at least some of the drawbacks of the prior art. The aligning tool and method may also be used for other surgical procedures, such as examination of a kidney tissue, or any other procedure requiring aligning a tissue plane with a reference plane, such as a cutting plane.